1. Field of the Invention
The present invention relates to an improved method and apparatus for determining the contact angle of a liquid droplet on a substrate surface upon which it is deposited.
2. Description of the Prior Art
Contact angle measurements of liquid droplets on substrate surfaces commonly are used to measure wettability of the substrate surface by a liquid and to evaluate adhesion. The contact angle is defined as the angle between the substrate support surface and the tangent to the profile of the droplet at the point of contact of the liquid droplet with the substrate. The value of the contact angle of the liquid droplet will depend upon substrate wettability. If perfect or complete wetting takes place between the liquid and the substrate surface by reason of high surface energy, the droplet will spread out over the substrate surface and the contact angle will approach 0 degrees, whereas if wetting is only partial, the resulting contact angle will lie in the range of 0 to 180 degrees. Devices are known for determining the contact angle of the droplet, both by direct measurement of the angle and by indirect calculation based upon measurements of the height, width, and/or radius of the droplet. Most common procedures involve projecting a silhouette image of the deposited droplet onto a projection screen and determining the contact angle by direct or indirect measurements taken from the silhouette.
Direct measurement of the contact angle is achieved by first establishing a tangent to the profile of the silhouette at the contact point with the substrate and then measuring the angle between the tangent line and the surface of the substrate. This method is subject to significant error. Accurate positioning of a tangent line is difficult since establishing the tangency of a line to an arc is very subjective. As a result, errors as high as 6 degrees can occur.
Indirect measurement of the contact angle can be made by calculation based upon measurements of the droplet silhouette. This method is applicable when the substrate surface is smooth and homogeneous and the droplet is very small, on the order of 10 microliters or smaller, so that the droplet takes the shape of a spherical segment and the distorting effect of gravity is negligible. In such case, the contact angle .THETA. can be calculate EQU tan .THETA./2=H/R (1),
where H is the drop height and R is the radius of the drop base. EQU .THETA.=2.times.arctan H/R (2).
After the ratio H/R is calculated, the contact angle value can be found in a specially prepared look-up table. This multi-step process is not only time consuming, but is also a subject to error, since the overall chance of error increases with each step of the process.